The present invention relates to a ferromagnetic powder suitable for use in forming a magnetic layer of a coating type magnetic recording medium such as a magnetic tape and a magnetic disk.
In a so-called coating type magnetic recording medium comprising a magnetic layer formed on a support by applying a coating film of a magnetic powder dispersed in a binder resin, there are demands for a still higher recording density. To cope with this requirement, ferromagnetic powders consisting of finer particles are being used, and with decreasing the size of the particles, improvements have been made in the electromagnetic conversion properties such as the output, the C/N ratio, frequency characteristics, etc. For instance, in Japanese Patent.
Publication JPA-Hei-6-36265 (Nos. 36265/1994), JPA-Hei-6-163232 (Nos. 163232/1994), JPA-Hei-7-331310 (Nos. 331310/1995), JPB-Hei-7-74365 (Nos. 74365/1995), JPA-Hei-7-126704 (Nos. 126704/1995), JPA-Hei-7-179913 (Nos. 179913/1995) and JPA-Hei-8-181008 (Nos. 181008-1996) are proposed various types of ferromagnetic powder which constitute the magnetic layer of the coating type magnetic recording medium. These disclosures also show the magnetic properties as well as the powder characteristics of the ferromagnetic powders.
Furthermore, it is desired to make the magnetic layers thinner to obtain high output characteristics while minimizing noise. To achieve this requirement, there is proposed a coating type magnetic recording medium having a multilayer structure, in which a non-magnetic layer is provided as a coating film of a non-magnetic powder dispersed in a binder resin, between the magnetic layers and the support. For instance, in U.S. Pat. Nos. 5,496,622 and 5,637,390 is disclosed a magnetic recording medium having the multilayer structure, and as a magnetic powder for forming the magnetic layer, there is described the use of fine acicular metallic powder of iron containing Co, a rare earth element, Al, Si, etc.
In obtaining a magnetic recording medium increased in density by using a fine-grained magnetic powder, regardless of whether it has a multilayer structure or not, the following problems were found to be overcome:
(1) The saturation magnetization decreases with reducing size of the particles, thereby leading to a magnetic recording medium having low output and a low C/N ratio;
(2) The coercive force similarly decreases with reducing size of the particles, thereby leading to a magnetic recording medium having low output and a low C/N ratio;
(3) The desired particle shape cannot be maintained with reducing size of the particles, thereby leading to a magnetic recording medium having low C/N ratio due to the particles deformed in shape or round particles, or to the presence of pores in the particles; and
(4) The weather resistance tends to be impaired with increasing saturation magnetization.
According to an aspect of the present invention, there is provided a ferromagnetic powder containing iron as the principal component and containing more than 5 to 50 at. % Co, 0.1 to 30 at. % Al, 0.1 to 10 at. % of a rare earth element inclusive of Y, 0.05% by weight or less of an element belonging to Group 1a of the Periodic Table, and 0.1% by weight or less (inclusive of 0% by weight) of an element belonging to Group 2a of the Periodic Table, said powder comprising flat acicular particles having a mean major axis length of 0.01 to 0.40 xcexcm and a crystallite diameter as determined by X-ray diffraction (Dx) of 50 to 250 angstrom, provided that the cross section diameter in the minor axis direction cut perpendicular to the major axis has a larger width and a smaller width, and that this cross section ratio in the minor axis direction, which is a larger width to smaller width ratio, uniformly yields a value of greater than 1, preferably, a value of 1.5 or higher, and said powder yielding a "sgr"s/Dx ratio of 0.7 or higher, wherein, "sgr"s represents the saturation magnetization and Dx represents the crystallite size determined by X-ray diffraction.
The ferromagnetic powder according to the present invention preferably releases H2O at 100xc2x0 C. at an amount of 2% by weight or less, and releases H2O released at 300xc2x0 C. at an amount of 4% by weight or less; it preferably has a true density thereof is 5.55 g/cm3 or higher and the specific surface area as determined by BET method is in a range of from 30 to 70 m2/g; it preferably yields a saturation magnetization ("sgr"s) in a range of from 100 to 200 emu/g and a coercive force in a range of from 1,200 to 3,000 Oe; and it has a decrease in saturation magnetization ("sgr"s) of 15% or lower when left under an atmosphere with a relative humidity of 90% at 60xc2x0 C. for one week.